Method of forming silicon compound fibers

ABSTRACT

A process for producing low-density high-modulus fibers including the steps of inserting silicon metal, preferably in powdered form, in a glass tube to form a composite, vacuum casting to melt the silicon metal in the tube, drawing the resulting glass-silicon composite rod into a fiber having a filamentary silicon core and an outer glass sheath, removing the glass sheath and contacting the exposed silicon core with a suitable reactant to form a silicon compound fiber.

O United States Patent [151 3,640,693 Galasso et al. 1 Feb. 8, 1972 [54]METHOD OF FORMING SILXCON 3,362,803 1/1968 Dannohl et a1 ..65/13COMPOUND FIBERS 3,368,871 2/1968 O'Connor et a1. .23/208 A 3,413,707121968 Kl tal ..29 419 [72] Inventors: Francis S. Galasso, Manchester;Richard 3 455 745 7x969 it; 81 29204 East 3,483,072 12/1969 Cox et a1..65/13 [73] Assignee: United Aircraft Corporation, East Hartford, Conn.Primary Examiner-S. Leon Bashore Assistant Examiner-Robert L. Lindsay Fl 3 9 [22] l ed Feb 196 Att0rneyJohn D. Del Ponti [21 l App]. No.:795,783

[57] ABSTRACT [52] US. Cl ..65/2, 23/191, 23/204, A process f producinglowdensily high modu|us fib 23/208, 29/419, 6 1 3 cluding the steps ofinserting silicon metal, preferably in powdared f i a glass t be to forma composite, vacu n. e o m i ing to melt the silicon metal in the tube,drawing the resulting 23/!91 209 208 208; 129 glass-silicon compositerod into a fiber having a filamentary silicon core and an outer glasssheath, removing the glass [56] References Cited sheath and contactingthe exposed silicon core with a suitable UNITED STATES PATENTS reactantto form a silicon compound fiber.

3,352,637 11/1967 Heymer et al. ..23/191 8 Claims, 2 Drawing FiguresPAIENIEnrw 8 m2 CHUCK VACUUM CHUCK [N VENTORS FRANCIS S. GALASSO RICHARDD. VELTRI \akmm ATTORNEY METHOD OF FORMING SILICON COMPOUND FIBERSBACKGROUND OF THE INVENTION This invention relates to the production oflow-density highmodulus fibers and more particularly relates to a methodof forming silicon compound fibers characterized by the absence of aforeign metal core.

In recent years considerable effort has been expended in the preparationof low-density high-modulus fibers for use in lightweight compositestructures. In particular, fibers of boron, and fibers of siliconcarbide, as well as fibers of silicon carbide coated boron, havedemonstrated the potential of fulfilling the stringent needs ofaerospace application. The production of these fibers is carried out, ingeneral, by a process wherein a suitable gas is pyrolytically decomposedon a heated filamentary tungsten core with the deposit being built up tothe desired thickness. One of the major drawbacks in such a processresides in the fact that with an alien metal substrate, such astungsten, whose density is high relative to the material beingdeposited, the fibers produced have an effectively higher resultantdensity by virtue of its presence. It, of course, would be advantageousto minimize fiber density by replacing or doing away with the tungstencore, particularly in aerospace application where weight savings resultdirectly in increased payload, but until now this could not effectivelybe done without jeopardizing desirable fiber properties. In addition,the cost of a tungsten core process as the one above described appearsalmost prohibitive particularly when compared to the potential savingsof the instant invention. Not only is the rate of fiber formationrelatively slow in a pyrolytic deposition process, but the tungstenneeded is available only at substantial and high costs.

SUMMARY OF THE INVENTION In the instant invention, there is disclosed amethod of fortning low-density, high-modulus silicon compound fiberswithout the necessity of utilizing a foreign metal core. The methodincludes the production of such fibers as silicon carbide, siliconnitride and boron silicide by the chemical transformation siliconfilament.

According to one aspect of the invention, there is provided a method ofconsistently reproducing, on a continuous basis, low-density,high-modulus silicon compound fibers amenable to usage as reinforcementmaterials. There is provided a process for the production of siliconcompound fibers by casting silicon metal as the core of a glass-metalcomposite rod while inhibiting reaction between the silicon core and itsouter glass shell, drawing the composite rod into a composite fiber,removing the shell from the fiber, and contacting the exposed siliconcore with appropriate materials under reaction conditions as desired.

The process includes the vacuum casting of the silicon in a pretreatmenttechnique according to a predetermined heating schedule prior to thefiberization of the composite rod.

BRIEF DESCRIPTION OF THE DRAWINGS An understanding of the invention willbecome more apparent to those skilled in the art by reference to thefollowing detailed description when viewed in light of the accompanyingdrawings, wherein:

FIG. 1 is an elevational view, partly in section, of the composite rodand associated apparatus illustrative of the pretreatment step of theinventive process; and

FIG. 2 is an elevational view, partly in section of the composite rodand associated apparatus illustrative of the fiberizing step of theinventive process.

DESCRIPTION OF THE PREFERRED EMBODIMENTS Referring now to the drawings,wherein like numerals indicate like parts; commercially availablesilicon l0, usually in powder form, is inserted into a glass tube 12,preferably a capillary tube, and is pretreated in a manner to render thesilicon amenable to subsequent fiberization. The composite rod is heatedto the fiberization temperature of the glass tubing and is drawn by anysuitable fiber drawing means (not shown) such as a rotating drum, toproduce a glass-sheathed silicon composite fiber of the desired size.The glass sheath is then removed by passage of the composite through anetching solution of hydrofluoric acid and the exposed silicon filamentis subjected to an appropriate gas under reaction conditions to form asilicon compound fiber.

In a preferred embodiment of the invention, the glass tubing 12 isclosed at its lower end, such as by a plug 14, is filled with siliconpowder 10 and is connected at its upper end to a vacuum pump 16. Afeeder mechanism such as a chuck I8 is provided to continuously feed thecomposite rod to a furnace 20 at a predetermined rate in order to vacuumcast the silicon core within the glass tube. It is to be understood thatthe vacuum casting of the silicon is a pretreatment step which iscarried out without causing a disturbance to the glass.

During experimentation, it was found that silicon powder does not meltuniformly during the drawing of the composite and that pretreatmentthereof is necessary to the formation of a satisfactory silicon fiber.In order to produce acceptable silicon filaments, it was furtherdetermined that a vacuum casting of the silicon within the glass tubinghad to be made prior to the fiberization of the composite. The preciseconditions under which the silicon is to be cast depends, to someextent, on the type of glass selected for use as the tubing 12. Theglass tube can be of any conventional fiber-forming glass compositionhaving a fiberizing temperature above the melting point of silicon(1,4l0 C.) but below that temperature at which the silicon reacts withits glass host. Pyrex composition glasses with a drawing temperaturearound 1,200" C. were found unsuitable since that temperature is too lowto allow melting of the silicon prior to composite fiberization.Additionally pure silica glass was found unsuitable since its drawingtemperature of 2,000 C. is too high and results in reaction with thesilicon.

After due investigation, it was found that a glass such as the OwensCorning Glass Co. glass sold under the name Vycor and having a 96percent silica content and a drawing temperature of about 1,850 C. wasmost compatible with both silicon pretreatment and composite drawing.Accordingly, Vycor glass capillary tubing 12 having an outside diameterranging from 6 to 9 mm. and an inside diameter ranging from 3 to 5 mm.was used during tests. During pretreatment, the vacuum pump 16 wasutilized to reduce and maintain the core pressure at 10 torr. Thesilicon was vacuum cast by passing the composite rod through a hot zoneproduced by the heater 22. It was determined that passage through a hotzone maintained at 1,550 C. at a rate of 30 cm./hour gave best results.Zone melting at a faster rate, for example at 40 cm./hour, fails toproduce a uniform casting while zone melting at a slower rate, forexample at 20 cm./hour, results in a Vycor-silicon reaction. The zonemelting range was thus established as being at approximately 25-35cm./hour, and preferably at 30 cmjhour through a zone heated to l,550 C.

Following the vacuum casting of the silicon core, the glasssiliconcomposite is heated to the fiberization temperature of the glass tubing12. As indicated previously, this temperature is above the melting pointof the silicon core 10 so that both the glass and the silicon will flow.As shown in FIG. 2, the composite is preferably lowered into afiberizing furnace 22 by a feeding mechanism 24 such as a chuck, locatedvertically above the furnace. When the fiberizing process is initiated,the plug 14 is removed and the lower end of the glass tubing 12 isclosed by heating. There is thus produced a length of glass only beforethe production of the glass-silicon fiber.

The furnace 22, as well as the furnace 20, can be of any type, such asan open or partially closed induction furnace as long as it achieves thedesired heating levels and contains an open portion through which theglass-silicon composite can be passed. The temperature range to whichthe composite is heated in furnace 22 depends of course on thefiberization temperature of the particular glass tubing being utilizedin the process. This temperature range must be high enough to permitcomposite drawing but again must be low enough to prevent glass-siliconreaction. With the preferred Vycor glass tubing, the useful fiberizationrange at which satisfactory composite filaments can be drawn has beendetermined to be from l,865 to l ,890 C.

After the proper fiberization temperature of the glass shell 12 isachieved, the composite is drawn by conventional means such as a drawingdrum (not shown) to form a continuous glass fiber 26 containing a coreof filamentary silicon. The speed at which the drawing step is carriedout can be varied to produce composite fibers of varying sizes. Ofcourse, the final diameter of the fibers depends not only on the drawingspeed but also on the viscosity of the melt and the initial size of thecomposite. In general, drawing speeds of about 50 to 500 feet per minuteare used to produce a composite fiber whose silicon core has a diameterof from 1 to 4 mils.

Following the attenuation of the composite by drawing, the glass sheathis suitably removed by passing the fiber through a normal HF etchingsolution (60 percent concentration) to expose the silicon core filament.The silicon filament is then passed through a reaction chamber such asfor example the chamber shown in the copending application Ser. No.618,512 filed by Basche et al. on Feb. 24, 1967 which shares a commonassignee with the instant invention. The fiber is exposed to a reactantgas in the chamber which transforms the silicon to a silicon compound.In a preferred embodiment, the silicon filament is heated in a carboncontaining gas such as methane at atmospheric pressure in a temperaturerange of l,2001 ,410 C. to accomplish conversion to silicon carbide.Other reactants can be used to produce other silicon compounds. Ammoniaor boron trichloride, for example, can be used to produce fibers ofsilicon nitride (Si N and boron silicide (B Si) respectively.

The degree of conversion of the fiber from silicon to silicon compoundwill of course be influenced by the thickness of the silicon startingfilament. Most of the silicon compound filaments produced to date havebeen homogeneous, that it totally converted, with a small remainderbeing only substantially converted. It should be noted, however, thateven if total conversion is not achieved, the remaining core ofunreacted silicon is not disadvantageous since, from a weightstandpoint, it is lighter than a foreign metal core such as tungsten.

Illustrative of the present invention, the following example is given:

A length of high temperature Vycor glass capillary tubing having anominal diameter of 6 mils (ID. of 3 mils and CD. of 6 mils) was closedoff at its lower end and filled with approximately 6 inches of siliconpowder. The upper end of the tubing was then attached to a vacuum pumpand evacuated to 10' torr. The glass-silicon composite was attached to avertical feeding mechanism and positioned so that the closed lower endof the tube was in the hot zone of an open inductionheated furnace at atemperature of 1,550 C. The tube was passed through the hot zone at 30cm./hour so that the silicon melted and then solidified to form a vacuumcast core.

The pump and plug were disengaged from the glass-case silicon compositeand the composite was then attached to a second vertical feedingmechanism above a second open induction-heated furnace. The compositewas positioned so that its lower end was in the hot zone of the furnaceat a temperature of 1,865-l ,890 C. When the outer glass tubing reachedthe furnace temperature, a fiber was drawn from its lower end at a speedof 300 feet per minute. The resulting composite fiber had a diameter of5 mils.

The composite fiber was passed through a 60 percent solution of HF sothat the glass sheath was etched away to expose a silicon core 2.5 milsin diameter.

The silicon filament was heated in methane at l,400 C. at atmosphericpressure and was totally converted to silicon carbide. This was verifiedby X-ray diffraction analysis.

It can be seen that by the present invention a process has beendiscovered wherein silicon compound fibers are produced by continuouslydrawing filaments of silicon as the core of a glass-silicon compositefiber, removing the glass sheath and contacting the exposed silicon withthe appropriate reactant. The unique method of manufacturing reinforcingfibers in accordance with the present invention provides an economicaltechnique heretofore unknown.

What has been set forth above is intended primarily as exemplary toenable those skilled in the art in the practice of the invention and itshould therefore be understood that, within the scope of the appendedclaims, the invention may be practiced in other ways than asspecifically described.

What is claimed is:

1. A process for producing low-density, high-modulus silicon filamentscomprising the steps of:

inserting silicon metal in a glass tube to form a composite rod;

vacuum casting to melt the silicon metal in said glass tube;

heating the resulting composite rod to the fiberizing temperature of theglass tube; said fiberizing temperature being above the melting point ofthe silicon metal and said glass tube being substantially inert to saidsilicon metal at said fiberizing temperature;

drawing the composite rod into a composite fiber comprising afilamentary core of silicon sheathed in a glass envelope; and

removing said envelope to expose said silicon filament.

2. The process described in claim 1 wherein said silicon filament iscontacted with a reactant to form a silicon compound fiber.

3. The process described in claim 2 wherein said reactant is methane anda silicon carbide fiber is formed.

4. The process described in claim 2 wherein said reactant is ammonia anda silicon nitride fiber is formed.

5. The process described in claim 2 wherein said reactant is borontrichloride and a boron silicide fiber is formed.

6. The process described in claim 1 wherein said vacuum casting isachieved by passing the composite rod through a hot zone of l,550 C. ata speed of 30 cm./hour.

7. The process described in claim 1 wherein the fiberizing temperatureof the glass is within a range of l,865 to 1,890 C.

The process described in claim 7 wherein said glass tube has a silicacontent, by weight, of approximately 96 percent.

2. The process described in claim 1 wherein said silicon filament iscontacted with a reactant to form a silicon compound fiber.
 3. Theprocess described in claim 2 wherein said reactant is methane and asilicon carbide fiber is formed.
 3. The process described in claim 7wherein said glass tube has a silica content, by weight, ofapproximately 96 percent.
 4. The process described in claim 2 whereinsaid reactant is ammonia and a silicon nitride fiber is formed.
 5. Theprocess described in claim 2 wherein said reactant is boron trichlorideand a boron silicide fiber is formed.
 6. The process described in claim1 wherein said vacuum casting is achieved by passing the comPosite rodthrough a hot zone of 1, 550* C. at a speed of 30 cm./hour.
 7. Theprocess described in claim 1 wherein the fiberizing temperature of theglass is within a range of 1,865* to 1,890* C.